The marker pops up and completely disappears waaay before the boss starts casting the attack. It's pretty quick to appear and disappear too! Then again, this could be a graphical glitch. From what everyone has said, the glitch may either be, 1) a display bug in which you can attempt to restart the game client to try to fix it, or 2) facing the camera at certain angles will cause the mark to not appear (kind of like flat surfaces at a straight angle). Not sure if this is affecting everyone - I can see it just fine and some others have stated the same as well.